Treatment of unbleached sulfite pulp



Oct. 19, 1954 R, McEwEN ET AL 2,692,181

' TREATMENT OF UNBLEACHED SULFITE PULP Filed March 8, 1950 2 Sheen's-Sheet l 0 /z34'5-@7ao-/0 Z 70m ALKAL/ (CALCULATED lVa 0/1) INVENTORS Robe/z A. Ncfu/en and BY Fred H. She/don ATTMNEYJ Oct. 19, MQEWEN ET AL TREATMENT OF UNBLEACHED SULFITE PULP 2 Sheets-Sheet 2 Filed March 8, 1950 m H P w wwwwwwm% INVENTORS Robe/r L. Mail/e17 and BY free R. She/dun Patented Oct. 19, 1954 TREATMENT or UNBLEACHED sULFrrE PULP Robert L. McEwcn, Williamsviile, and Fred -R. Sheldon, Buffalo, N. Y, assignors to Buffalo Electro-Chemical Company, Tonawanda, N. Y.

Application March 8, 1950, Serial No. 148,312

4 Claims.

The present invention relates to a method of treating unbleached sulfite wood pulps to achieve moderate to high brightness fibers which possess unimpaired or improved physical properties as, for instance, tensile strength, burst, tear and cuprammonium vicosity.

Sulfite wood pulp is produced from wood chips by removing therefrom substantially all of the non-cellulosic matter by cooking the chips in the presence of sulfite type chemicals in order to loosen, dissolve and remove most of the lignin and non-cellulosic materials from the cellulose. The resultant fibers are generally designated as clean, good forming, medium or strong fibers depending on the cooking procedure and by reason ofthese desirable properties are employed for specific purposes where strength, formation, and cleanliness is ultimately required in the end product prepared from the fiber. In other words, these fibers are employed in the production of higher grades of paper products than the softer, shorter and lower strength so-called mechanical fibers, that is, fibers which contain substantially all of the normal ingredients of the wood itself.

In the production of high grade paper products from high grade chemical fibers, it is very often desirable that the chemical fiber be bleached or brightened to enhance the value and utility of the pulp. The procedures normally availed of for producing brightened fibers from sulfite pulp are normally to bleach the sulfite fiber either with chlorine compounds or alkaline peroxide.

The former process is presently the only bleaching operation commercially employed upon sulfite pulps. This procedure sufiers disadvantages, the greatest of which is the cellulose degradation produced in the sulfite pulp by reason of the employment of the chlorine bleach to obtain the high brightness levels desired in bleached suliite fibers. Furthermore, the chlorine bleach on sulfite fibers is normally a complicated multi-stage process; as the chemical residues and reaction products thereof must be removed, the chlorine bleach entails a considerable amount of washing between stages and after the last stage; therefore,

extensive equipment employing large quantities of water and utilizing considerable amount of floor space is required. A resultant disadvantage arises by reason of the fact that there is not only a loss of fiber by the action of the purification and bleaching chemicals but also a high mechanical loss by reason of the multi-stage treatment with attendant washing. The multi-stage processes, consisting of chlorination and hypochlorite bleaching, with or without alkaline extraction stages and washes between these steps, are necessary in order to reach high brightness levels with a minimum of cellulose degradation. But in so doing, yield losses of 5 to 10 occur and considerable equipment is involved.

Some grades of sulfite pulp are bleached with hypochlorites or hypochlorous acid in a single stage; however, said resulting pulps are generally of moderate brightness; or if of high brightness, are of lower strength and low yield due to the excessive amounts of chlorine used.

The heretofore suggested operations for bleaching sulfite pulp with peroxide have not been accepted commercially. This is probably by reason or" the fact that such processes have required multi-stage purification and bleaching which reduces yield and like the chlorine multistage operations, require washing steps, and extensive equipment and plant space.

No single stage commercially acceptable sulfite pulp bleaching procedures for reaching high brightness levels with high yield and without cellulose degradation have yet been presented in this field, using either chlorine or peroxide, as the efiective bleaching ingredient.

It is an object of the present invention to provide a method for peroxide bleaching of sulfite pulps wherein mechanical loss by washing is materially decreased and ancillary thereto, materially to decrease and, in efiect, eliminate the problem of the disposal of the'highsolids content wash waters.

It is also an object of the invention to provide a single stage peroxide bleaching method for the production of high brightness sulfite pulps using a minimum of equipment and operating area.

It is also an object of the invention to produce bleached sulfite pulps of unimpaired physical properties and with strengths superior to those possessed by pulps brightened to the same degree in the conventional single or multi-stage chlorine systems.

It is also an object of the invention to provide a method of bleaching sulfite pulps for the production of high strength, high brightness fibers.

It is also an object of the invention to provide a method for bleaching sulfite pulps wherein the yield of bleached pulp is substantially Conventional methods for determining aggregate fiber strength of pulps are to determine burst, fold, tear, and tensile strength of handsheets of the fibers prepared according to set standard procedures. For determining individual fiber strength, the viscosity of cellulose dispersions measured in centipoises is usually used as a measure of fiber quality. For this system; either 3 dispersions of cellulose in cuprammonium solution or in cupraethylene diamine are used. In the instant case, cuprammonium viscosities have been used to determine comparative fiber degradation.

In the drawing,

Fig.1 is a graph plotting the viscosities of a peroxide bleached sulfite pulp against various per cent total alkali calculated as N aOH is used in our preferred single stage alkaline peroxide bleaching process.

Fig. 2 is a graph plotting viscosities of an alkaline peroxide bleached sulfite pulp against the bleaching temperatures.

In the broad aspect of the invention, the unbleached sulfite pulp is treated under conditions producing the maximum brightness, with the employment of a minimum of peroxide and attendant alkali and peroxide stabilizer, and under conditions where the physical and chemical damage to the pulp is at a minimum. These conditions are obtained at a high pulp density during the bleach. When the pulp density during alkaline peroxide bleaching is below that at which water ceases to be the continuous phase, the attainment of the desirable high brightness levels necessitates the use of excessive quantities of I .chemicals and results in increased pulp damage. The bleaching by our preferred procedure is conducted therefore at pulp densities above about and preferably in the range to 65% "in order to reduce damage to the pulp.

'A second factor which must be controlled is the chemical load employed per unit of unbleached I pulp. As seen in Fig. 1, this has been represented as per cent total alkalinity expressed as NaOH.

In other words, only that quantity of total alkali should be employed as is required to produce the alkaline environment required for the peroxide bleach and substantially no excess, as excess of alkali materially reduces the 'cuprammonium viscosity in the resultant bleached sulfite pulp.

A third factor which produces pulp damage when unbleached sulfite pulps are treated with alkaline peroxide is elevated temperature. See Fig. 2. Where the temperature of the bleach .impregnated pulp mass is below about 130 F., little damage can be determined and ascribed to the temperature elfect. However, where the temperature of the bleaching mass of fibers is raised to 150 or 180 F., the pulp damage is greatly increased when a specified amount of alkaline peroxide is used as shown by a decrease in the cuprammonium viscosity of samples of the bleached pulp. This is shown graphically in Fig. 2.

The ideal conditions of operation, therefore,

' in accordance with the present invention, are the treatment of unbleached sulfite pulp at pulp densities of 20 to 65%, and preferably between and 50%, with 0.3% to 1.75% of actual hydrogen peroxide (100% basis) and a total alkalinity of 0.75% to 3.25% based on fiber and calculated as NaOH at a temperature below 130 F. and preferably in the range 50 F. to 110 F.

' The bleaching may be performed on crumb vor on a self-sustaining lap of the proper pulp density. The mechanical difliculty of obtaining uniform bleachant addition to high density pulp crumbs makes it preferable to efiect the bleaching upon a lap of fibers and to this end a feltless wet ,machine of the Kamyr or Rodgers type is recommended, although any machine producing a lap of 80.0 and physical tests at The alkaline peroxide bleach is applied to the pulp lap or web, or crumbs in any conventional fashion, as by the employment of a spray in the case of crumbs or by spray or padding rolls in the case of a web. The quantity of solution added may be relatively small to obtain the required chemical ratio on the fiber, so that the pulp density is reduced by such impregnation to only a slight degree. The fibers with added alkaline peroxide are then maintained damp for the duration of the bleach, which at the preferred low temperature may require from about 1 to 20 days to reach the high brightness level desired. Generally the level will be reached in 2 to 14 days.

The fibers impregnated with the alkaline peroxide are maintained damp by piling the crumbs or stacking or rolling the laps or webs in close contact to permit slow and even bleaching to occur under the conditions where the pulp is maintained in the desired density range and fiber damage maintained at a minimum. Thereafter, the bleached fibers may be employed as required for paper furnishing.

The following examples are given as merely illustrative of the invention rather than limitative thereof.

EXAMPLE I Spruce sulfite pulp having a brightness of 55.1 (as measured with the G. E. photoelectric colorimeter), was bleached by impregnating a web at 40% density with an alkaline bleaching solution so that the fibers contained 1% hydrogen peroxide basis), 6% sodium silicate (41 B.), and 1% caustic soda, all on the basis of bone dry pulp. The impregnated lap was of 37% density and was thereafter accumulated into rolls, and allowed to bleach at 70 F. In one day, the pulp brightness was 76.4; in three days, 80.8, and in six days had reached a maximum of 82.3 G. E. brightness.

An example of a conventional peroxide bleaching system carried out on the same pulp, where water was the dominating phase is the following: The same pulp was bleached at 12% density, using 1% hydrogen peroxide, 6% silicateof soda and 1.5% caustic soda, an optimum formula for this density. Maximum brightness was obtained after 4 hours at 55 0., and was found only to be 66.1 Hunter brightness.

EXAMPLE II Spruce sulfite pulp of 54.9 brightness was bleached to a brightness of 79.2 G. E. by our preferred method, by impregnating pulp with 0.9% hydrogen peroxide (100% basis), 4.5% silicate of soda, 0.7% caustic soda, all on a bone dry pulp Weight basis, and allowing the so impregnated pulp to stand moist at room temperature for 11 days. Strength testsare given in Table 1.

In contrast, a standard two-stage chlorine bleach on similar spruce sulfite had a brightness maximum burst, as reported in Table l:

standard two-stage chlorine bleach reduced the Mullen and tensile below that of the unbleached sample, the bleached product of the present invention possessed a better Mullen and tensile than the unbleached sample. The tear was considerably better than that of the unbleached EXAMPLE III Sulfite pulp of 59.4 Hunter brightness was formed into a web and bleached by our preferred method, by impregnating a 40% density lap with a peroxide solution so that the pulp contained hydrogen peroxide, 6% sodium silicate, and 1.0% caustic soda, all on an oven dry pulp weight basis, and thereafter maintained at 36% density for 14 days. Standard handsheets were made and brightness was found to be 82.4.

Another sample of the pulp processed in exactly the same way but using hydrogen peroxide was bleached to a brightness of 78.6 (Hunter refiectometer) The solution for brightening the sulfite pulp in accordance with the present invention is an alkaline peroxide solution. Such solution may be made from an alkali metal peroxide, such as sodium peroxide, by neutralizing the excess of alkali with a mineral acid, such as sulfuric acid, or by adding alkali, such as caustic soda, to aqueous hydrogen peroxide. Although these will be the materials commercially employed, it will be understood that the inorganic peroxides generally are applicable as are the perborates, i. e., those substances which produce hydrogen peroxide in aqueous solution. As in the general employment of peroxide solutions, a so-called stabilizer will be present and is preferably a soluble silicate such as sodium silicate, although other stabilizers may be employed such as soluble salts of magnesium, as well as certain alkali phosphates and alkali pyrophosphates. The requisite total alkalinity will be derived from sodium peroxide when that chemical is employed as the source of hydrogen peroxide, and from the alkali associated with the stabilizer. If this be insufficient, then caustic soda or soda ash may be added as desired.

By peroxide is meant hydrogen peroxide, sodium peroxide, and generally peroxides which in aqueous solution produce hydrogen peroxide.

Within the sense of this invention, all types of sulfite pulps may be utilized, Whether they are produced from soft woods or hard woods, and whether they are cooked with calcium, magnesium, ammonium or mixed base liquors, or whether they are so-called soft, medium, or hardcooked pulps.

With more particular reference to the drawings, Fig. 1 shows the decrease in the cuprammonium viscosity and, therefore, in the fiber strength of a sulfite pulp bleached through a range of total alkali from about 0.5% to 9% total alkali calculated as NaOH.

A series of samples of 44.2 grams oven dried pulp were formed into sheets and sprayed with a bleach to give 1% H202 on the oven-dried pulp. The consistency was adjusted to 35% and the pulp sheet permitted to bleach in the damp condition; at maximum brightness (14 days), the cuprammonium viscosity of the pulp was deter- "6 mined. The results are set forth in the table. The bleach solution ingredients on an oven dried basis were 1% H202, 4% silicate of soda and caustic alkali to give the indicated alkalinity. Of the percent total alkalinity calculated as NaOH, 0.54 is alkali attributable to sodium silicate and the remainder due to caustic alkali.

Total Oentipoises With reference to the graph in Fig. 2, .a series of samples were bleached at approximately 35% consistency with 1.5% H202, 6% sodium silicate and 1% caustic soda, based on oven dried pulp weight at temperatures ranging from 50 F. to 212 F. and the cuprammonium viscosity of the samples determined after maximum brightness had been reached. The average viscosity of the blank was 101 centipoises. These results are set 1. The method of treating a paper furnish consisting of unbleached sulfite pulp to produce bleached sulfite pulp without adversely affecting the fiber strength of the pulp, which comprises adding to the unbleached sulfite pulp sufiicient of an alkaline peroxide solution to obtain a fiber density in the range 20% to 65% and to produce a chemical content on dry pulp basis as follows: H202, 0.3% to 1.75%; total alkali as NaOH, 0.75% to 3.25%, and maintaining the fibers damp and at a density above 20%, for an extended period of time on the order of 1 to 20 days while maintaining the fiber temperature below F.

2. The method of treating a paper furnish consisting of unbleached sulfite pulp to produce bleached sulfite pulp without adversely affecting the fiber strength of the pulp, which comprises adding to the unbleached sulfite pulp sufficient of an alkaline peroxide solution to obtain a fiber density in the range of 20 to 65% and to produce a chemical content on a dry pulp basis as follows: H202, 0.75% to 1.25%; total alkali as NaOH, 1.0% to 2.5%, and maintaining the fibers damp and at a density above 20% for an extended period of time on the order of 1 to 20 days while maintaining the fiber temperature below 130 F.

3. The method of treating a paper furnish consisting of unbleached sulfite pulp to produce bleached sulfite pulp without adversely affecting the fiber strength of the pulp, which comprises adding to the unbleached sulfite pulp sufficient of an alkaline peroxide solution to obtain a fiber density in the range 20% to 65% and to produce a chemical content on a dry pulp basis as follows: H202, 0.75% to 1.25%; total alkali as NaOH, 1.0% to 2.5%, and maintaining .the fibers damp and at a density above 20% for an extended period of time on the order of 2 to 14 days while maintaining the fiber temperature below 130 F.

4. The method of bleaching a paper furnish consisting of sulfite pulp which comprises adding to the unbleached sulfite pulp furnish sufficient of an alkaline peroxide solution to dampen the fibers therewith and to obtain a fiber density above 20% and to produce a chemical content on dry pulp basis as follows: I-IzO2-0.3% to 1.75%; total alkali as"NaOH0.75% 'to- 3.25%,

and then maintaining these-treated fibers damp and at a density above 20% for an extended period of time on the order of 1 to 20 days while maintaining the fiber temperature below 130 F. during such'period whereby to produce a bleached sulfite pulp of substantially unimpaired physical characteristics.

' References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,107,297 Kaufimann Feb. 8, 1938 2,150,926 Kaufimannu Mar. 21, 1939 2,189,378 Kaufimann Feb. 6, 1940 2,465,738 McEwen Mar. 29, 1949 2,466,633 Brabender Apr. 5, .1949 2,492,047 KBurg et al Dec. 20, 1949 2,510,595 McEwen June 6, 1950 2,513,344 McEwen July 4, 1950 2,514,503 McEwen et al.' Ju1y 11, 1950 FOREIGN PATENTS Number Country Date 573,341 Germany Mar. 31,1933

July 13,1943.

Serial No. 356,113, Gartner (A. P. 0.), published April 27, 1943. 

1. THE METHOD OF TREATING A PAPER FURNISHED CONSISTING OF BLEACHED SULFITE PULP TO PRODUCE BLEACHED SULFITE PULP WITHOUT ADVERSELY AFFECTING THE FIBER STRENGTH OF THE PULP, WHICH COMPRISES ADDING TO THE UNBLEACHED SULFITE PULP SULFFICIENT OF ALKALINE PEROXIDE SOLUTION TO OBTAIN A FIBER DENSITY IN THE RANGE 20% TO 65% AND TO PRODUCE A CHEMICAL CONTENT ON DRY PULP BASIS AS FOLLOWS: H2O2, 0.3% TO 1.75%; TOTAL ALKALI AS NAOH, 0.75% TO 3.25%, AND MAINTAINING THE FIBERS DAMP AND AT A DENSITY ABOVE 20%, FOR AN EXTENDED PERIOD OF TIME ON THE ORDER OF 1 TO 20 